Making dimethyl sulfide from pulp mill spent liquors



Dec. 17, 1957 M. E. ClSNEY ETAL I 2,816,832

MAKING DIMETHYL SULFIDE FROM PULP MILL SPENT LIQUORS Filed Sept. 1'7, 1954 PULP MILL SPENT LIQUOR PREFERABLY 'CONCENTRATE TO 30-60"/. ,BY WEIGHT SOLIDS INSURE THAT LIQUOR IS ALKALINE, PREFERABLY HAVING A pH OF AT LEAST IO ADD FROM A TRACE TO I593 BY WEIGHT SULFUR, BASED ON SOLIDS CONTENT OF WASTE LIQUOR DISTILLA DlNIET-HYL SULFIDE INVENTORS MERLE E. CISNEY y JAMES D. WETHERN ATIORNEY MAKING DllVIETHYL SULFIDE FROM PULP MILL SPENT LIQUORS Merle E. Cisney and James D. Wethern, Camas, Wash.,

assignors to Crown Zellerbach Corporation, San Francisco, Califl, a corporation of Nevada Application September 17,1954, Serial No. 456,693

12 Claims. (Cl. 92-2) This invention relates to a process for making dimethyl sulfide from residual liquors derived from the pulping of lignocellulose.

Dimethyl sulfide is a useful or potentially useful material in many applications, for example, as an odorant for addition to odorless gases to allow their detection or tracing, as a low boiling solvent, and as a raw material in the synthesis of various organic derivatives, such as dimethyl sulfoxide, dimethyl mercaptan and ternary sulfonium compounds. Its use in these various applications is substantially restricted, however, by its high cost, which currently is about $1.00 per pound.

' It obviously would be desirable to make dimethyl sulfide from pulp mill spent liquors, which contain components convertible to dimethyl sulfide and which are available in very large volume. However, in the conventional alkaline pulping processes the material balance in the cooking liquor is controlled between carefully defined limits and any procedure proposed for making dimethyl sulfide from the spent pulping liquor isnot economically feasible if it destroys this balance either by the addition orsubtraction of its component chemicals. For example, the process could not beused successfully, if .it were to add extraneous sodium so that the sodium concentration of the liquor is built up to an inordin'antly high level. Similarly, it would not be successful if .sulfur were withdrawn from the liquor until its sulfur concentration had reached a level so low as to destroy the sulfur balance of the process.

Accordingly, it is the general object of this invention to provide a process for making dimethyl sulfide from pulp mill spent liquor which may be applied without destroying the material balance employed in the pulping procedure.

It is another object of this invention to provide a process for making dimethyl sulfide from pulp mill waste liquor which may be incorporated into the conventional pulping procedures without the necessity of modifying the material flow or thevinclusion of elaborate special equipment.

It'is-another object of this invention to provide a process for making dimethyl sulfide from pulp mill waste liquor which produces a valuable chemical as a by-product of the pulping industry, thereby making possible reduction in price of the pulp product,

It is another object of this invention to provide a process formakingdimethyl sulfide from pulp mill waste liquor which will produce at low cost a very large volume of dimethyl sulfide, making it available for its numerous potential applications. 1 Y

The present invention is predicated upon the discovery that when elemental sulfur is reactedundercontrolled conditions with pulp mill' spent liquor, it reacts with the constituents.thereot presumably with the ligninmethoxyls, to produce substantial yields of'dimethyl sulfide.

mains then may be treated further, and its constituent ed States PatentO 2,816,832 Patented Dec. 17, 1957 chemicals recovered .and reused in the conventional manner.

. Considering the presently described process in greater detail and with particular reference to the drawing comprising a flow plan of the instant procedure:

The pulp mill waste liquor which is the starting material for the presently described process may be derived from the pulping of any of the conventional lignocellulose raw materials by any of the conventional pulping procedures. The lignocellulose accordingly may be derived from the coniferous species of trees, particularly Douglas fir and hemlock, from the deciduous species of trees, particularly alder or cottonwood, and from other plant materials such as straw.

The pulping procedures employed may be the conventional kraft procedure utilizing sodium hydroxide and sodium sulfide, the soda process utilizing sodium hydroxide as the sole pulping agent, or the sulfite processes, using calcium, magnesium, ammonium, or sodium bases under acid, alkaline or neutral conditions.

In the kraft process, lignocellulose is cooked in an aqueous medium with a solution of sodium hydroxide and sodium sulfide having a sulfidity of from 5 to 40 percent, the sulfidity being defined as the ratio of the amount of sodium sulfide to the total amount of sodium hydroxide and sodium sulfide in the liquor, both constituents being expressed as sodium oxide.

The lignocellulose is cooked in the above reaction mixture for from 1 to 8 hours at a maximum temperature of to 190 (3., and a maximum pressure of from 30 to p. s. i. At the conclusion of this operation the pressure is usually reduced and the cellulosic pulp separated from the black cooking liquor. The latter is concentrated in evaporators and burned to generate steam as well as to recover for reuse the chemicals used in the cooking process.

process are dissolved in water. The resulting gree'n liquor then is causticized with lime to convert the sodium carbonate in the smelt to sodium hydroxide, the calcium carbonate produced as a by-product being precipitated andremoved. The resulting white liquor then is ready for use in another cooking cycle.

The soda pro-cess is similar to the above with the exception that sodium hydroxide, with or without a small amount of added sodium sulfide, is used as the pulping agent. The sulfidity of the cooking liquor is from 0 to 5 percent, the cooking time from 2.5 to 20 hours, the maximum cooking temperature from 135 to C., and the maximum cooking pressure from 30 to 170 p. s. i. At the conclusion of the cooking operation, the cellulosic pulp is separated from the black liquor which then may be processed, as described above for steam generation and recovery of the chemicals contained in the liquor.

The general analyses of the black liquor sol-ids for the kraft and soda processes are as follows:

In'both ofthe foregoing pulping processes small losses of sodium salts and of sulfur occur during-the evaporation and combustion of the black liquor. These losses conventionally are compensated for in the kraft process by "adding sodium sulfate to theliquor. This is reduced 'in' the furnace by the carbon present in the mixture to form sodium sulfide. In the soda process, the loss is compensated for by adding. make up sodium hydroxide directly to the white liquor.

In both processes it is highly important that the percentage of active chemicals in the cooking liquor, as well as the relative sodium and sulfur balances, be maintained constant so that a uniform yield and quality of pulp may be produced. Accordingly, where it is desired to process the black liquor for the recovery of other materials, as the recovery of dimethyl sulfide by the present process, it is essential and critical that the processing operation should not disturb these balances.

In the sulfite pulping procedures lignocellulose, usually in the form of wood chips, is cooked with ammonium, sodium, calcium or magnesium bisulfites under acid, neutral or alkaline conditions. In the conventional acid sulfite process where the cooking liquor consists of an aqueous solution of bisulfite together with an excess of sulfur dioxide, the pH of the liquor is from 1.5 to 5. In the so-called bisulfite process, where a smaller proportion of sulfur dioxide is employed, the pH of the liquor is about 5-6. In the neutral and alkaline sulfite processes, an alkaline buffer is added to the liquor so that its initial pH is from 7 to 12, depending upon the amount of alkali used. When the initial pH is of the order of 10 to 12 the pH of the resultant spent liquor then may be from 7.5 to 9.

The cooking of the lignocellulose with the conventional sulfite liquor is eifectuated at a temperature of from 120 to 155 C. and a pressure of from 65 to 120 p. s. i. for a period of from 6 to 24 hours. At the end of this time, the pressure is reduced and the cellulose pulp separated from the spent liquor.

This liquor comprises essentially lignin in the form of salts of ligninsulfonic acids, these being present to the extent of about 50 to 75 percent by weight, solids basis. In addition, there is present from 15 to 35 percent, solids basis, of wood decomposition products, principally carbohydrates, and from 6 to 10 percent by weight of ash, principally the sulfites and sulfates of calcium, magnesium and sodium. Where, however, the cooking medium is ammonium bisulfite, the ash content is about 1 percent by weight.

The spent sulfite liquor may be processed for the recovery of its content of pulping chemicals and this is done commercially, in some instances. It, like the black liquor rcesulting from the kraft and soda processes, is the starting material for the presently described procedure for making dimethyl sulfide.

In executing the process of the present invention it may be desirable to subject the spent liquor to one or more preliminary processing operations. If the liquor is acid in character, as is the case with some of the sulfite waste liquors, it is made basic, preferably to a pH of at least 10. Since in the usual case its solids content is from 10 to by weight, it preferably is concentrated to provide a solids content of from and 60% by weight. This facilitates the procedure since it is not necessary to handle such large volumes of liquid and also since it reduces the hold-up of dimethyl sulfide in the reaction liquor, dimethyl sulfide having an appreciable solubility therein at the reaction temperature.

The alkaline agent, if any is employed for adjustment of the pH of the liquor, may comprise any of the alkaline materials conventionally used for this purpose, such as the oxides, hydroxides, or carbonates of the alkali metals. These may be added with stirring until the desiredpI-I level has been achieved. Similarly, the concentration of the liquor to the desired solids content may be efiectuated, using any suitable equipment such as the conventional evaporation vessels which may or may not be operated under vacuum.

After the spent liquor has been subjected to these preliminary treatments for adjustment of its pH and solids content, it is treated with a predetermined quantity of elemental sulfur preferably employed in the form of sulfur powder. Broadly stated, sutficient sulfur should be added to provide the 'stoichiometric quantity of that element required for reaction with the methoxyl groups present in the lignin, since it is believed that these groups are responsible for the ensuing production of dimethyl sulfide.

Although the quantity of sulfur added accordingly is somewhat variable, depending upon the nature and identity of the spent liquor employed, from a trace to 15% by weight of sulfur, based on the weight of the spent liquor solids, generally is suitable. Any amount of sulfur, i. e. a trace, will convert some of the spent liquor to dimethyl sulfide.

On the other hand, any amount over about 15% by weight is in excess of the theoretical amount required to accomplish the desired conversion, and also may complicate the reaction by producing undesirable by-products through reaction of the sulfur with the sulfides and mercaptans which may be present in the liquor, yieldingpolysulfides and disulfides respectively, as well as with the lignin itself to form complex products of indeterminate character.

Thus it has been found that where kraft black liquor is the starting material, from /2 to 4% by weight of elemental sulfur preferably is used; where soda liquor is the starting material, from 1 to 7% by Weight sulfur represents a preferred range; and where sulfite liquor is used, from 1 to 10% by weight sulfur may be used to advantage.

The spent liquor and sulfur are reacted in a suitable reaction vessel under reaction conditions calculated to provide the maximum yield of dimethyl sulfide in minimum time with minimum production of undesirable by-products. The reaction vessel may be of any suitable construction, with or without means for agitating the contents. Preferably, it is of closed construction so that the reaction may be effectuated under pressure. Also it should be provided with means for heating the contents to a temperature of between and 500 0, preferably between 180 and 300 C. The pressure maintained within the vessel ranges between atmospheric and that of an entirely closed system comprising waste liquor and sulfur. Although the pressure obviously will vary depending upon the composition of the waste liquor, in a typical case it may reach a value of from 400 to 1200 p. s. i.

The reaction is carried on under the above reaction conditions until it is substantially complete, i. e. until the sulfur has reacted substantially completely with the lignin methoxyl groups. The time required is variable depending principally upon the temperature and pressure employed. In the usual case, however, it requires from 2 seconds to 120 minutes, preferably from 3 to 60 minutes, the longer reaction time being applicable to situations wherein the less strenuous reaction conditions are employed and vise versa. An extremely short reaction time, for example, a few seconds, may be employed when a continuous flash operation is contemplated, as in a reaction vessel heated to the upper ranges of the indicated reaction temperatures.

The dimethyl sulfide product preferably is removed by venting the vapor from the reaction mixture into a condenser in a closed system. The vapor is condensed and provides a two-phase product comprising dimethyl sulfide and water. The dimethyl sulfide layer then may be separated, dried, and/or fractionally condensed to purify it.

The presently described process is illustrated by the following examples:

Example 1 5.7 kilograms of kraft black liquor derived from the pulping of a mixture of Douglas fir and western hemlock and having a solids content of 46.8% by weight and a pH of 12.6 was placed together with 40 grams elemental amass sulfur in an autoclave and heated with stirring to a temperature of 250 C. The temperature was maintained with continued stirring at a level of 250 to 254 C. for a reaction period of 10 minutes, during which time the pressure rose from 680 p. s. i to 820 p. s. i.

A vaporous reaction product was vented from the sys tem during the next 11 minutes into a condenser and collected in a cooled receiving vessel. The condensed product separated into two layers, the upper dimethyl sulfide layer being decanted, dried over anhydrous sodium sulfate, and filtered. It weighed 87.5 grams, representing a yield of 3.26% by weight, based on the solids content of the black liquor. This crude product then was purified by distillation, the distilled fraction weighing 77 grams, a yield of 2.87%, based on the solids content of the black liquor. It was colorless, insoluble in water, but miscible with most organic solvents. It boiled at 3638 C. had an index of refraction of 1.436 at 24 C. and a density of 0.845 at 26 C.

Example 2 In a manner similar to that outlined in Example 1, 4,680 grams of cottonwood kraft liquor having a pH of 12.4 and a solids content of 19.6 percent was treated with 22.5 grams of elemental sulfur for the recovery of dimethyl sulfide. The reaction was carried out at a temperature of 224-227 C. for a time of 11 minutes. The yield of dimethyl sulfide was 1.25 percent by weight.

Example 3 Kraft black liquor resulting from the pulping of Douglas fir was concentrated to a solids content of 46.5 percent. 5.7 kilograms of the concentrated liquor having a pH of 12.2 then was heated with 40 grams elemental sulfur at 215220 C. for 26 minutes. The dimethyl sulfide was isolated as in Example 1, and weighed 52.6 grams. This represented a 2.0 percent by weight yield, based on the black liquor solids.

Example 4 30 grams sulfur was added to 3750 grams Douglas fir soda liquor having a solids content of 18.9% and a pH of 13.0. The mixture was heated under pressure with stirring at a temperature of 226-232 C. and a pressure of 480 p. s. i. for a time of 12 minutes. The reaction vapors then were vented and collected as in Example 1. The condensed dimethyl sulfide represented a yield of 0.7% based on the solids content of the waste liquor solids.

Example 5 Western hemlock ammonium base sulfite waste liquor was concentrated to a solids content of 45%. It then was alkalized with sodium hydroxide pellets to a pH of 11.5. 20 grams of elemental sulfur then were added to 2800 grams of the concentrated waste liquor and the mixture heated with stirring at a temperature of 236-240 C. and a pressure of 600 p. s. i. or a time of 6 minutes. The reaction vapors then were vented through a condenser to a receiving vessel placed in an ice bath and the dimethyl sulfide collected in this manner. After separation from the water and drying, it represented a yield of 0.67%, based on the sulfite waste liquor solids content.

Hence it will be apparent that by the present invention we have provided a process for making dimethyl sulfide from a raw material readily available in large quantities, viz. pulp mill spent liquor, which produces a substantial quantity of dimethyl sulfide at very low cost. For example, in a kraft mill producing 400 tons of paper pulp per day, the presently described process would produce about 30,000 pounds per day of dimethyl sulfide at a cost which is only a small fraction of the current price of dimethyl sulfide.

Furthermore, the presently described procedure can be incorporated into the system for recovering chemical materials from pulp mill waste liquors without substantial change in material balance and without the necessity of including elaborate equipment. Since elemental sulfur alone is added to the system, and this sulfur is removed therefrom in the form of dimethyl sulfide, the overall sulfur or sodium content in the system is maintained at a desired level for continuity of plant operation. Also, no extraneous materials such as sodium are added which would result in excessive build-up of such material, leading to high viscosity of solutions with attendant pumping and burning difliculties.

Having now described our invention in preferred embodiments, we claim:

1. The process of making dimethyl sulfide from spent liquor resulting from the alkaline pulping of lignocellulose and having a pH of at least 10, the process comprising adding to the liquor, as the sole essential reactant, from about V2 to 15% by weight, based on the liquor solids, of elemental sulfur, heating the resulting reaction mixture at ISO-500 C. for a time sufiicient to produce a substantial quantity of dimethyl sulfide, and separating the dimethyl sulfide product from the reaction mixture whereby a liquor having substantially the same amount of pulping agent remains.

2. The process of claim 1 wherein the spent liquor is concentrated to a solids content of from 30-60% by weight before it is heated with the elemental sulfur.

3. The process of claim 1 wherein the reaction mixture is heated at a temperature of from 180 C. to 300 C.

4. The process of claim 1 wherein the amount of elemental sulfur added is not substantially in excess of the stoichiometric amount required for reaction with the methoxyl content of the lignin compounds present in the liquor.

5. The process of claim 1 wherein the spent liquor comprises kraft spent liquor.

6. The process of claim 1 wherein the spent liquor comprises soda spent liquor.

7. The process of claim 5 wherein the total amount of sulfur present in the liquor is not substantially in excess of the stoichiometric amount required for reaction with the methoxyl content of the lignin compounds present in the liquor.

8. The process of making dimethyl sulfide from the spent liquor resulting from the alkaline pulping of lignocellulose and having a pH of at least 10, the process comprising: adding to the liquor, as the sole essential reactant, from about /2% to 15 by weight, based on the liquor solids, of elemental sulfur; heating the resulting reaction mixture at 180-300" C. and 400-1200 p. s. i. for a time of from 3-60 minutes; and separating the dimethyl sulfide product from the reaction mixture whereby a liquor having substantially the same amount of pulping agent remains.

9. The process for making dimethyl sulfide which comprises cooking lignocellulose in an aqueous medium containing an alkaline inorganic lignocellulose pulping agent, separating the resulting cellulosic pulp from the residual spent liquor, said liquor having a pH of at least 10, adding to said liquor from about /2 %to 15% by weight, based on the liquor solids, of elemental sulfur, heating the resulting mixture at from C. to 500 C. for a time sufiicient to produce a substantial quantity of dimethyl sulfide, separating the dimethyl sulfide from the reaction mixture, evaporating and burning the spent liquor from which the dimethyl sulfide has been removed, thereby recovering the pulping agent, and applying the recovered pulping agent to pulping of a further quantity of lignocellulose while maintaining the material balance of the lignocellulose pulping medium.

10. The process of claim 9 wherein the pulping agent comprises a mixture of sodium hydroxide and sodium sulfide.

References Cited in the file of this patent UNITED STATES PATENTS 130,114 Dupont Aug. 6, 1872 1 5, 1943, 996,225 Drewsen June 27, 1911 10 1,249,287 Warte et al. Dec. 4, 1917 1,626,171 Wells Apr. 26, 1927 York (1943). 1,714,831 Tingl May 28, 1929 8 Marshall et' a1 Aug. 10, 1954 Hagglund June 21, 1955 FOREIGN PATENTS Germany Aug. 25, 1925 OTHER REFERENCES Ser..No. 429,343, Carlander (A. P. C.), published June Cellulose and Cellulose Derivatives, by Ott; pp. 506 and 507; published by Interscience Publishers, Inc., New 

1. THE PROCESS OF MAKING DIMETHYL SULFIDE FROM SPENT LIQUOR RESULTING FROM THE ALKALINE PULPING OF LIGNOCELLULOSE AND HAVING A PH OF AT LEAST 10, THE PROCESS COMPRISING ADDING TO THE LIQUOR, AS THE SOLE ESSENTIAL REACTANT, FROM ABOUT 1/2 TO 15% BY WEIGHT, BASED ON THE LIQUOR SOLIDS, OF ELEMENTAL SULFUR, HEATING THE RESULTING REACTION MIXTURE AT 150-500*C. FOR A TIME SUFFICIENT TO PRODUCE A SUBSTANTIAL 